This invention relates to a drive apparatus for a specimen stage carrying specimens to be measured (observed) and more particularly to a specimen stage drive apparatus suitable for handling specimens of known shape or known internal structural configuration in a microscope such as an electron microscope or an optical microscope.
When observing a specimen of precedently known shape or internal structural configuration with an electron microscope or the like, it is conventionally widespread practice that a drawing or photograph illustrative of the specimen is used and brought into association with the movement of a specimen stage in some way in order that a part, desired to be observed, of the specimen can be moved quickly and easily into a very small field of view of the microscope. The specimen part desired to be observed is very small and hence the view field of the microscope is also very small measuring several of tens of microns to several mm.
One of conventional methods which are considered to be highly effective for moving a specimen to be observed into such a small field of view is disclosed in literature (1) of an English catalogue "IEEE COMPUTER SOCIETY REPRINT, ELECTRON BEAM TESTER WITH 10 ps TIME RESOLUTION, pp 601-606, ISSUED AT PROCEEDINGS OF INTERNATIONAL TEST CONFERENCE, Sept. 8 to 11, 1986" (especially, p 601, FIG. 3). A similar technique is disclosed in literature (2) entitled "High Speed E-Beam Tester", NIPPON GAKUJUTSU SHINKOHKAI, No. 132 IINKAI, No. 93 KENKYUKAI SHIRYO, Nov. 1985, pp 40-45 (especially p 43).
FIG. 1 is a schematic diagram illustrating a prior art drive apparatus for a specimen stage of a microscope as taught from either of the above literatures.
Referring to FIG. 1, a specimen stage 1 carrying a specimen 4 can be moved in two orthogonal directions (X-axis direction and Y-axis direction) by means of stepping motors 2 and 3 driven under the direction of a controller 5 comprised of a CPU. A mechanical coordinate system (Xs, Ys) for the specimen stage 1 is managed by the controller 5. When a desired position on the planar surface of a coordinate digitizer 6 constituting a two-dimensional coordinates designating means is designated by means of a stylus pen 7, the coordinate digitizer 6 delivers to the controller 5 coordinate values on plane coordinate system (Xd, Yd) which are representative of the position designated by the stylus pen 7. The coordinate digitizer 6 is typically equivalent to an input unit widely used in a computer and will not be detailed herein. The coordinate digitizer 6 is commercially available having the coordinates output planar surface which measures a size of about A4 to a larger size of A0. As shown in FIG. 1, a manual switch 9 is connected to the controller 5 and the stepping motors 2 and 3 are driven by the controller 5 during depression of the manual switch 9. More particularly, the stepping motor 2 is driven to move the specimen stage 1 in the horizontal direction (X-axis direction) while a horizontal push button 9x of the manual switch 9 is being depressed, and the stepping motor 3 is driven to move the specimen stage 1 in the vertical direction (Y-axis direction) while a vertical push button 9y of the manual switch 9 is being depressed. By depressing the horizontal and vertical push buttons 9x and 9y simultaneously, the specimen stage 1 can be moved in an oblique direction.
While the specimen 4 is placed on the specimen stage 1, a drawing 8 illustrative of an internal structural configuration of the specimen 4 is placed on the coordinate digitizer 6. For example, when the specimen 4 is a semiconductor integrated circuit, a copy on a reduced scale of a layout design figure of the circuit may be used as the drawing 8. If the specimen 4 could be placed accurately at a predetermined position on the specimen stage 1, and the drawing 8 could be placed accurately at a predetermined position on the coordinate digitizer 6 and the ratio between sizes of specimen 4 and drawing 8 could be determined accurately, the correspondence between the coordinate system (Xs, Ys) for specimen stage 1 and the coordinate system (Xd, Yd) on the coordinate digitizer 6 could be determined. Actually, however, it is almost impossible to accurately place the drawing 8 at a predetermined position on the coordinate digitizer 6 and to accurately determine the size ratio between specimen 4 and drawing 8. Under the circumstances, the correspondence between the two coordinate systems needs to be somehow set up and this can be accomplished in the following manner.
(1) If the specimen 4 is exemplified as an integrated circuit, a specified point on the circuit, for example, a corner of a bonding pad (point A in FIG. 2) is moved to the center of view field of the microscope by using the manual switch 9 and a corresponding point (point A' in FIG. 2) in the drawing 8 on the coordinate digitizer 6 is designated using the stylus pen. Through this operation, the controller 5 reads and stores mechanical coordinates (Xs.sub.1, Ys.sub.1) for the specimen stage 1 and coordinates (Xd.sub.1, Yd.sub.1) on the coordinate digitizer 6.
(2) Subsequently, a different specified point on the specimen 4, for example, a corner of another bonding pad (point B in FIG. 2) is moved to the center of view field of the microscope by means of the manual switch 9 and a corresponding point (point B' in FIG. 2) in the drawing 8 is similarly designated by the stylus pen 7. The controller 5 then reads coordinates (Xs.sub.2, Ys.sub.2) and coordinates (Xd.sub.2, Yd.sub.2) of the two points B and B'.
(3) Using four of the thus read coordinates (Xs.sub.1, Ys.sub.1), (Xd.sub.1, Yd.sub.1) and (Xs.sub.2, Ys.sub.2), (Xd.sub.2, Yd.sub.2), the controller 5 calculates, for the sake of setting up the correspondence between the coordinate system (Xs, Ys) for specimen stage 1 and the coordinate system (Xd, Yd) on the coordinate digitizer 6 expressed as EQU Xs=K (Xd cos.theta.-Yd sin.theta.)+Xs.sub.0 ( 1) EQU Ys=K (Yd cos.theta.+Xd sin.theta.)+Ys.sub.0 ( 2),
parameters which are expressed as ##EQU1## where .theta. appearing in equations (1), (2) and (3) represents a relative angle between specimen 4 and drawing 8 as can be seen from FIG. 2. FIG. 2 explains the principle of coordinate conversion by illustrating the relative positional relationship between specimen 4 and drawing 8.
After completion of the above procedure, the controller 5 utilizes coordinate values on coordinate system (Xd, Yd) designated by the stylus pen 7 and delivered out of the coordinate digitizer 6 to calculate coordinate values on coordinate system (Xs, Ys) for the specimen stage, thereby driving the specimen stage 1.
In this manner, by manipulating the stylus pen 7 adapted to designate a desired position on the drawing, a corresponding point on the specimen 4 can be moved readily and quickly into the view field of the microscope.
Incidentally, in an application, the specimen measurement through the use of a microscope conducts itself with a view to comparing a plurality of specimens 4 having an identical structural configuration. For example, the apparatus disclosed in the literature (1) quoted hereinbefore measures voltage waveforms appearing in an integrated circuit and requires comparative measurement between an integrated circuit operating abnormally and an integrated circuit of the same configuration operating normally for the sake of rapid fault analysis. In the apparatus using an electron beam, a specimen is placed in vacuum and it takes a very long time to complete vacuum evacuation, requiring that desirably, non-defective and defective specimens to be compared with each other be placed on a specimen stage at a time. In such an instance, only either one of the two specimens, though the two specimens being designed to have exactly the same configuration, is permitted to be brought into correspondence with a layout drawing illustrative of the specimen.
This problem can be solved by accurately placing two identical layout drawings on the coordinate digitizer in exactly the same positional relation as that of two specimens on the specimen stage but this manner of solving is very difficult to achieve.